Immunodepression, a Noncontributor to Radiation-induced Leukemogenesis of the RFM Mouse¹

Following a single acute exposure to 300 R of X-rays at 6 weeks old, approximately 65% of female RFM mice die of thymic lymphoma during the first year of life. In contrast, nonirradiated animals do not die of this neoplasm during this same period. To determine if immediate immunological restoration is of significance in interrupting the inductive process, we injected 50 × 10⁶ syngeneic spleen or bone marrow cells into these animals immediately following X-irradiation. Restoration of immunocompetence as measured by both humoral and cell-mediated parameters was more rapid in spleen cell-reconstituted animals than in bone marrow-treated animals; however, spleen cells failed to protect the mice against the irradiation-induced thymic lymphomas. In contrast, although there was no significant difference in the immunological recovery of bone marrow-reconstituted animals and animals that received only irradiation, mortality was dramatically reduced as a result of marrow injection. Therefore, although immunodepression is an inherent component, it cannot be considered as a critical obligatory requirement in the pathogenesis of radiation-induced thymic lymphomas of RFM mice.